(1) Field of the Invention
The present invention relates to a light scanning device and an image forming apparatus using it, and in particular relates to a light scanning unit including a polygon mirror for deflecting a plurality of light beams by reflection with an identical facet thereof; and an optical system for irradiating a plurality of illumination targets, arranged in the auxiliary scan direction, with corresponding beams, as well as relating to an image forming apparatus such as a copier, printer, facsimile machine etc., which includes the aforementioned light scanning device and uses electrophotography as the image forming method thereof.
(2) Description of the Prior Art
Recently, image forming apparatuses for supporting high-speed printing jobs have been being developed.
For example, the processing ability of a conventional image forming apparatus used to be 40 to 60 sheets per minute in terms of the number of printing with standard paper (A4 short-edge feed) a few years ago, but at present the processing speed has been enhanced up to as high as 100 to 120 sheets per minutes.
In the image forming apparatus including a light scanning device (light scan unit) for forming an image by irradiating the photoreceptor with beams, it is necessary to radiate beams on exact positions in order to obtain high-quality printing. However, high-speed image forming apparatus configurations suffer the problem of increased vibrations due to rotations of motors for the apparatus and rotations of apparatus drivers, etc., during the operation of the machine.
Particularly, as the factor of losing irradiation performance of beams in the light scanning unit, vibrations of optical parts can be considered. In a system that performs deflection scanning using a polygon motor, the polygon motor is prone to be the source of vibrations because it rotates at high speed.
Since the light scan unit has a fixed optical path length for performing a write operation by radiating the beam from the light source on the predetermined position of the photoreceptor, the positions of arrangement of multiple mirrors for reflecting the illuminated beam are definitely determined depending on the positional relationship between the position of the polygon motor and the photoreceptor in the apparatus.
If a vibration arises from the polygon motor or an apparatus driver in the light scan unit, vibrations take place (occur with predetermined amplitudes from the vibration source) depending on the material and thickness of the frame unit that constitutes the light scan unit. Therefore, it is important for improvement in writing quality to avoid the aforementioned multiple mirrors being placed at resonance points.
As the measures against vibrations in light scan units, some configurations have been proposed: for instance, Japanese Patent Application Laid-open Hei 09-236769 (patent literature 1) in which attachment portions of optical parts to the frame unit are optimized so as to arrange the optical parts at positions with lower vibrations; and Japanese Patent Application Laid-open 2001-208996 (patent literature 2) in which optical parts are laid out at positions away from the antinodes of vibrations.
However, it is true that the above methods are effective when the optical scan unit is small, but in a light scan unit of a tandem type where multiple beams are scanned simultaneously by a single polygon motor to perform write operations, the light scan unit as a whole becomes bulky and is composed of a large number of optical parts, hence it is no longer possible to deal with the vibrations, by a passive method such as arranging optical parts at positions with low vibrations or by laying out optical parts away from the antinodes of vibrations. So there is a demand for an active method of suppressing vibrations at the positions where optical parts are laid out.